1. Field of the Invention
The present invention relates to a sealed battery manufactured by coupling a cover member to an opening portion of a prismatic outer case and welding the cover member to the opening portion of the prismatic outer case and a method for manufacturing the same.
2. Description of the Background
Recently, because of an increasing demand for portable OA devices and communication devices there is a greater demand for a prismatic sealed battery as their power supplies. In those nickel metal hydride rechargeable batteries and lithium ion rechargeable batteries, a prismatic sealed batteries in particular can make the devices smaller in size and be mounted in the above-mentioned devices with high volume efficiency.
In general, the prismatic sealed battery has an outer case with an opening portion provided therein. In the battery, a cover member is coupled to the opening portion and a coupling portion between these members is hermetically welded by a laser beam.
The outer case has a power generating element with positive and negative polarities formed with a separator set therebetween and an electrolyte held therein. An electrode terminal is provided on the cover member to be electrically connected to the power generating element.
The cover member has an injection port for injecting an electrolyte into the outer case after the cover member has been welded to the outer case. The injection port is closed by a sealing cover after the electrolyte has been injected into the outer case. The sealing cover is welded there by a laser beam.
In the sealed battery thus constructed, when the injection hole in the cover member is closed with the cover and they are welded by a laser beam, the injected electrolyte in the outer case is evaporated by heat applied to the outer case, resulting in leaks out of the outer case. For this reason, the welded portion at the cover member and sealing cover is affected by the evaporated electrolyte, sometimes leading to the failure in forming a hermetical weld between the sealing cover and the cover member. In this case, the leaking electrolyte corrodes the cover member, etc., and mars the outer appearance, etc., of the battery.
Between the power generating element and the inner surface of the cover member a pressing member is provided for preventing the power generating element from being displaced in the outer case. Since the injection port is provided in the cover member, a hole is provided in the pressing member at a place corresponding to the injection port to allow the electrolyte to be injected from the injection port into the outer case or a cutout is provided in the pressing member along the shape of the port.
Although the hole or cut-out is provided in the pressing member, if the electrolyte is injected from the injection port into the outer case, there is sometimes the case where a portion of the electrolyte flows along the inner surface of the cover member and is accumulated at the injection port at a place between the cover member and the pressing member.
If, in such a state, the injection port is blocked by the sealing cover welded by a laser beam, the electrolyte is evaporated at an area between the cover member and the pressing member due to the ingress of heat. Due to this cause, there is sometimes the case where a hermetical weld cannot be performed at the welding section, leading to corrosion of the outer surface of the cover member followed by marring an outer appearance of the battery.